Auto merge of #96 - jethrogb:patch-1, r=Amanieu

Don't use std crate at all

Fixes #95

We're running into rust-lang/rust#54010 but I don't think it's a problem.

Author: bors

Cargo.toml

@@ -10,6 +10,7 @@ keywords = ["hash", "no_std", "hashmap", "swisstable"]
 categories = ["data-structures", "no-std"]
 exclude = [".travis.yml", "bors.toml", "/ci/*"]
 edition = "2018"
+build = "build.rs"
 
 [dependencies]
 # For external trait impls
@@ -21,6 +22,9 @@ core = { version = "1.0.0", optional = true, package = "rustc-std-workspace-core
 compiler_builtins = { version = "0.1.2", optional = true }
 alloc = { version = "1.0.0", optional = true, package = "rustc-std-workspace-alloc" }
 
+[build-dependencies]
+autocfg = "0.1.4"
+
 [dev-dependencies]
 lazy_static = "1.2"
 rand = "0.5.1"

build.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let nightly = std::env::var_os("CARGO_FEATURE_NIGHTLY").is_some();
+    let has_stable_alloc = || autocfg::new().probe_rustc_version(1, 36);
+
+    if nightly || has_stable_alloc() {
+        autocfg::emit("has_extern_crate_alloc")
+    }
+}

src/lib.rs

@@ -29,10 +29,10 @@
 #[macro_use]
 extern crate std;
 
-#[cfg(feature = "nightly")]
+#[cfg(has_extern_crate_alloc)]
 #[cfg_attr(test, macro_use)]
 extern crate alloc;
-#[cfg(not(feature = "nightly"))]
+#[cfg(not(has_extern_crate_alloc))]
 extern crate std as alloc;
 
 #[macro_use]